The invention is directed to a pressure-medium-actuated vehicle braking system having two brake circuits.
Such a vehicle braking system is shown in German Patent Document DE 195 04 393 C1. The known braking system is intended for a commercial vehicle and has a pneumatically and/or electrically operable rear-axle brake circuit for the two wheels of the rear axle and a pneumatically and/or electrically operable front-axle brake circuit for the two wheels of the front axle. In the event of a breakdown of one brake circuits, the other brake circuit should still sufficiently decelerate the commercial vehicle.
Two brake cylinders constructed as spring brake cylinders (combination brake cylinders) are assigned to the rear-axle brake circuit. The brake cylinders can be connected with a compressed-air reservoir for the rear-axle brake circuit by a first control valve device as a function of a control signal of a multiple-circuit braking value generator assigned to the rear-axle brake circuit. The front-axle brake circuit has a first brake cylinder for the right front wheel and a second brake cylinder for the left front wheel. These brake cylinders can be connected with a compressed-air reservoir for the front-axle brake circuit by a second control valve device as a function of a control signal of the multiple-circuit braking value generator assigned to the front-axle brake circuit. The second control valve device has a first pressure input connected with the compressed-air reservoir for the front-axle brake circuit and a second pressure input connected with a control output of a service brake valve, through which second pressure input a front-axle control pressure is admitted which is derived from the service brake valve. Furthermore, the second control valve device has a pressure output connected with the first brake cylinder and the second brake cylinder of the front axle brake circuits.
As a result of the dynamic axle load displacement during a braking operation, the major portion of the braking force has to be applied by the front wheels. This is true particularly in the case of commercial vehicles with a high front-axle load and an at least temporarily low rear-axle load, as, for example, in the case of semitrailer tractors. This results from a high dynamic axle load displacement which occurs when braking because of their short wheel base and their large center of gravity height. In the event of a failure of the front-axle brake circuit, it is therefore no longer ensured that the braking force, which is now supplied solely by the rear wheels, will be sufficient for achieving the deceleration specified by the legislator. Such a failure of the front-axle brake circuit is caused, for example, by the rupture of a compressed-air pipe or by leakiness in the front-axle brake circuit; so that in time, the pressure in the compressed-air reservoir of the front-axle brake circuit will fall.
In order to solve this problem, a reversing valve arrangement in the form of a relay valve is provided in known vehicle braking system between the pressure output of the second control valve device and the brake cylinder for the left front wheel. The relay piston of this relay valve can be pressure-controlled by a rear-axle control pressure derived from a multiple-circuit braking value generator such that the brake cylinder of the left front wheel can be connected with a compressed-air reservoir which is independent of the compressed-air reservoir of the front-axle brake circuit. The known relay valve has a double seat valve constructed as a combined inlet and outlet valve. When the front-axle brake circuit is intact, an effective area of the relay piston is acted upon by the front-axle braking pressure during braking, while the other effective surface is acted upon by the rear-axle control pressure. As a result, forces are generated at the relay piston which are directed against one another. The size of the effective surfaces of the relay piston is selected such that the outlet valve is held open during braking and thus the front-axle brake pressure generated by the second control valve device can be controlled through the opened outlet valve to the brake cylinder of the left front wheel.
In the event of a failure of the front-axle brake circuit, the pressure falls in the compressed-air reservoir assigned to the front-axle brake circuit and the front-axle brake pressure upon the one effective surface of the relay piston also falls. As a result, the relay piston switches the double seat valve into an emergency braking operating position because the relay piston is forced into a position closing the outlet valve and opening the inlet valve by the rear-axle control pressure still present at the other effective surface. The compressed air of the compressed-air reservoir, which is independent of the compressed-air reservoir of the front-axle brake circuit, can now be switched through the opened inlet valve to the brake cylinder of the left front wheel in order to be used there as a replacement for the failed front-axle brake pressure.
If, in addition, a wheel-slip-dependent control of the brake pressure takes place in the case of the known vehicle braking system, for example, for the purpose of the antilock control (ABS) or the wheel slip control (ASR), the brake pressure for each wheel or for each axle is temporarily controlled to zero. As a result, the above-described reversing valve arrangement would be switched into its emergency braking operating position and compressed air of the compressed-air reservoir, which is independent of the compressed-air reservoir of the front-axle brake circuit, would unnecessarily reach the brake cylinder of the left front wheel.
In contrast, the pressure-medium-actuated vehicle braking system according to the invention has the advantage that, in the event of a slip-controlled braking, a switching device, which is arranged in front of the reversing valve arrangement, switches into the blocking condition, in which no reversing signal can be switched through to the reversing valve arrangement. As a result, the unnecessary connection of compressed air from the additional compressed-air reservoir is eliminated. Because of the through-connection condition of the switching device in the case of a slip-free braking, the emergency braking function of the reversing valve arrangement is completely maintained.
A control device and automatic control device for the switching device contains a control unit by means of which, as a function of the slip control of the wheel assigned to the first brake cylinder, an electric control signal can be generated for controlling the switching device. The switching device may contain an electrically controllable 3/2-way valve which, when not energized, takes up the through-connecting condition and, when energized, takes up the blocking condition. Because of the currentless through-position of the 3/2-way valve, the reversing valve arrangement for triggering the emergency braking operation remains effective also when the control device and automatic control device fails or is switched off.